Abstract

Slip on gently dipping detachments in the brittle crust has been enigmatic for decades, because fracture mechanics laws predict frictional resistance is too great for sliding to occur, except under rather unusual circumstances. The Miocene Mormon Peak detachment in Nevada and the Eocene Heart Mountain detachment in Wyoming are two well‐studied examples of upper crustal, carbonate‐hosted low‐angle detachments, with highly debated slip processes. Both low‐angle faults were active during regional magmatism, and a number of proposed slip mechanisms involve magmatic fluids, frictional heating, or both. To address the role that magmatic fluids and frictional heating may have played in reducing friction, we measured clumped‐isotope ratios on 137 carbonate samples from these faults. The majority of fault breccias and gouges on the detachment slip surface record temperatures that are colder than the host rock. Surprisingly, samples from within 5 m of the Heart Mountain detachment average just 65 °C, and not a single sample (out of 37 measurements, excluding metamorphosed host rock at White Mountain) records a temperature greater than 90 °C. Along both faults, most samples are depleted in δ^(18)O relative to the host rock, indicating that meteoric, not magmatic, fluids were present and interacting with the fault rock. However, a few samples preserve temperatures of over 160 °C, which, based on textural and geochemical criteria, are difficult to explain other than by frictional heating during slip. These temperatures are recorded in one sample directly on the Mormon Peak detachment slip surface and in two hanging wall localities above the Heart Mountain detachment.